Current optical communications and communicators include line-of-sight communication systems in which modulated laser beams generated at one point are detected at a distant point, with the information carried on the modulated beam being decoded at the receiver location. In general, line-of-sight optical communication systems provide for wide band communications and, in general, are capable of carrying more information than conventional land line systems. However, such line-of-sight communication systems are typically utilized where there is either flat terrain and no structures interposed between the transmitter and the receiver, or in situations in which both the transmitter and the receiver are located well above the existing terrain. In general, the systems are, of course, horizon limited and, therefore, have a natural limitation to a range of approximately 20 miles.
While over-the-horizon radar has been developed which operates in a forward scatter mode, considerable problems exist with microwave forward scatter due to the frequency range in which these forward scatter systems operate and, indeed, to the overcrowding of the microwave bands. Moreover, the bandwidth of coherent optical communications systems exceeds that of microwave systems by an order of magnitude. There exists, therefore, a need for over-the-horizon optical communications systems operating in a forward scatter mode to make available the increased bandwidth at the optical frequencies. It will be appreciated that land lines have limited bandwidth typically on the order of 5KHz and that the cost of laying fiber optic cable is in most instances, prohibitive. Moreover, with the aforementioned line-of-sight optical communication systems, forward scatter is minimized so as to maintain an optimum signal-to-noise ratio.
Optical forward scatter communications have, in the past, been plagued by pulse stretching and smearing with the resultant bandwidth reduction, because scatter along the beam from many points to the receiver results in different path lengths between the transmitter and the receiver. Thus, different parts of a given pulse reach the receiver at different times, and this distorts the waveshape of the transmitted pulse, thereby destroying the information carrying capability of the communications link.
Scattering arises because of aerosols and particulate matter in the earth's atmosphere which results in the redirection of the collimated beam. Scattering typically occurs over 360.degree. and, therefore, it is possible for light scattered from one particle along the transmitted beam to arrive at the receiver with one path length while another particle, removed as much as 10,000 yards from the first point may scatter energy back to the receiver with a different path length and, thus, a different time of arrival. It is therefore possible, in a typical situation, to have a difference in arrival time of the same pulse on the order of dozens of microseconds. This results in the aforementioned smearing of the pulse such that it may overlap adjacent pulses and, thus, the interpulse spacing must be quite large in order to resolve the individual pulses. This, of course, considerably slows the data rate.
The subject system compensates for the smearing or pulse stretching which naturally occurs in the forward scatter of an optical beam by, in effect, dividing up the beam in the vicinity of the receiver into segments along the optical beam path. This is accomplished by utilizing a multiple field of view receiver having a number of detectors oriented parallel to the transmitted beam. The order of the signals from these detectors is inverted and the signals are then delayed by a suitable tapped delay line, such that the pulses from different segments exactly overlap and add. The delay from one signal to the next is set to compensate for different path lengths between the receiver and the transmitter such that pulse stretching or smearing is virtually eliminated.
Thus, in effect, the optical signal is segmented and reconstructed or reassembled with appropriate time delays which are the inverse of the delays encountered due to the difference in the optical path lengths. By virtue of the elimination of the pulse stretching or smearing, the subject system is capable of bandwidths of 10 MHz which makes the subject optical communication system suitable for video, telephony, FAX, and computer data transmission.
Another feature of the subject invention is one of noise rejection. With the subject optical path length compensator, an improved signal-to-noise ratio is achieved because noise will not exactly add at the various taps of the delay line, whereas the individual pulses which make up the communicated information will exactly add. Thus, while it is possible to get n x the signal at one detector for the pulses, the effective amplitude of the noise is .sqroot.n. The noise referred to is front end detector noise or noise generated by solar background.
With suitable and currently available high powered lasers, it is now possible to obtain forward scatter communications not only over and about obstacles in the straight line path, but also over distances of 100 miles or more. Because of the noise rejection feature of the subject system, solar radiation is not a problem and the subject system is operable both in the daytime, as well as night.
It is therefore an object of this invention to provide a forward scatter optical communication system in which path length distortion is eliminated.
It is another object of this invention to provide a method and apparatus for eliminating path length distortion in forward scatter optical communication systems while at the same time affording a certain measure of noise rejection.
It is a still further object of this invention to provide long range optical communication in which the bandwidth is optimized.
These and other objects of the invention will be better understood in connection with the detail description taken in conjunction with the following drawings.